Multi-band vertical antenna with concentric radiators



W. H. FOLEY Aug. 15, 1961 MULTI-BAND VERTICAL ANTENNA WITH CONCENTRIC RADIATOR-S IN VEN TOR.

Filed Dec. 10, 1957 FIG. I

WILLIAM H. FOLEY ATTOR N EYS United States Patent 2,996,718 MULTI-BAND VERTICAL ANTENNA WITH CONCENTRIC RADIATORS William H. Foley, New Boston, Mass., assignor, by mesne assignments, to Brunswick Sports Products Company,

a corporation of Delaware Filed Dec. 10, 1957, Ser. No. 701,873 2 Claims. (Cl. 343-825) This invention relates to a multiband antenna and more particularly to a multiband dipole in the form of a whip antenna.

It frequently happens in communication systems that there are frequencies in use in the system or network which are widely spaced from one another. For example, one might be concerned with frequencies in which the ratio in the range as far as wavelength is concerned might be of a factor of 3 to 1. It is known that in order to obtain efficient reception or transmission of signals that it is usually necessary to employ a half wave dipole that is placed the correct height above the earth and connected to the radio apparatus by appropriate transmission lines. Accordingly, when it is necessary to cover a range of frequencies that have a wavelength ratio of 3 to 1, it is impossible to build an antenna that will provide a reasonably good match to the transmission line and to the radio apparatus over the entire frequency range.

One solution to the problem is to arrange a number of half-wave dipole antennas in the form of a fan and connect them in parallel. Each dipole may then be tuned to one of the frequency bands that is being used. When such a scheme is employed, we first must recognize that we have at resonance, in a half-wave dipole, an impedance of about 70 ohms. Accordingly a suitable transmission line should have a characteristic impedance of 70 ohms. In such a system, the dipoles that are off-resonance will provide generally high and predominantly reactive impedances of both capacitative and inductive nature in parallel with the basic 70 ohms of the resonant dipole. From a practical standpoint, these parallel impedances have a rather small over all effect, but in general it can be said that at the common feed point of this multiple stack of dipoles there will be presented a lower impedance than 70 ohms, for instance a value somewhere in the 50 to 70 ohm range. With a 70 ohm transmission line, standing waves will therefore be present. From an impedance standpoint, a further departure may occur if harmonic operation is utilized in the various bands, for instance, suppose that three meter operation were desired and also ten meter operation. In this case when transmitting on three meters, the ten meter antenna will be close to resonance at its third harmonic and its center impedance will be low, perhaps something on the order of 100 ohms. Accordingly in a case such as this, the resultant impedance of the combined antenna system may be lowered considerably to' something on the order of 40 ohms. If per chance some of the other transmitting bands are also harmonically related such as on the lower frequency end, these particular sections will be close to harmonic resonance and their radiation currents will be in phase presenting a high impedance to the transmission line which would be essentially resistive. Conversely, the higher frequency section of the radiating system will present an impedance usually consisting of both resistive and capacity reactance which will also be high compared to the transmission line impedance. Accordingly, with these particular design considerations in mind over any range of frequencies, a proper match can be obtained from such a system.

It will be apparent, however, that if multiband operation is desired on a mobile vehicle, the use of dipoles Patented Aug. 15, 1961 becomes unwieldy. The solution, therefore, is to utilize a quarter-wave vertical radiator, also known as a whipantenna or marconi, and combine the antennas in such a manner that a compact structure is provided.

It is therefore one of the objects of this invention to construct an antenna which will be effective over a wide band of frequencies.

Another object of the invention is to construct an antenna as a single radiating member to cover a wide band of frequencies.

A more specific object of the invention is to construct a broad band, single radiating antenna of multiple radiating elements.

A still further object of the invention is to construct a single radiating element with multiple radiators utilizing a plurality of multiple length dipoles and connecting these dipoles to a common transmission line.

With these and other objects in view, the invention consists of certain novel features of construction as will be more fully described and particularly pointed out in the appended claims.

In the drawings:

FIGURE 1 is an elevational view of an antenna embodying the invention;

FIGURES 2, 3 and 4 are elevational views of the antenna embodying the invention showing the progressive steps during the construction thereof, and in which the diameter-to-length ratio has been greatly increased to illustrate the principle of the invention;

FIGURE 5 is a sectional view taken on line 5-5 of FIGURE 4;

FIGURE 6 is a schematic showing a possible method of utilization of the antenna; and

FIGURE 7 is a partial elevational view of the antenna on an enlarged scale showing the conductors therefor.

In proceeding with this invention and by way of example of one method of manufacturing the antenna, I take insulating material which may be impregnated with a suitable insulating resin and form this material into a suitable cross sectional shape. One suitable method of forming the insulating material is to wind the suitably impregnated material on a mandrel of a desired shape. The resulting structure is then placed in a braiding machine or winder and an electrical conductor may either be braided or wound thereon with a suitable pigtail being left at either end for later fabrication. Upon the conductor another layer of insulating material is formed, this material being also preferably suitably impregnated with a synthetic resin. A second electrical conductor is then braided onto the second insulating layer of material for a shorter distance than the first conductor and a suitable pigtail is left at one end for later fabrication. A third layer of insulating material is formed over this structure and a third electrical conductor of still shorter length is formed over the third layer of insulating material. The resulting structure is then covered with another layer of insulating material and the entire structure may be placed in an oven for curing the resin in accordance with techniques that are developed for the resin in use so as to establish a homogeneous structure. The pigtails left at the bottom or one end of the resulting structure are then joined together and connected through suitable means to a transmission line such as by means of a ferrule coupling and the other end of the initial wound electrical conductor is connected to an antenna tip consisting of a ball or other device to eliminate coronaend effect.

Referring to the drawings a complete antenna is shown in FIGURE 1 consisting of a homogeneous mass generally indicated '10 which include the radiating elements surmounted by a suitable corona ball 11 having at the lower end thereof a plurality of pigtails extending therefrom, generally indicated 12. The radiating portion of the antenna is composed of a core 13 preferably consisting of an insulating material impregnated with a suitable resinous material such as a phenolic. The actual insulating material utilized may of course take many forms, illustrative of which would be a resinous sheet material having reinforcing fibers, a woven material impregnated with resin suitable laminous resinates etc. This core 13 may assume any cross-sectional shape such as elliptical, cylindrical or polygonal and is preferably hollow although it may be solid. The core when it assumes a hollow configuration is preferably formed of a strip or sheet of material wound upon a removable mandrel such as M (FIGURE 5), the strip or sheet of material having previously been coated or impregnated with an insulating resin. The core 13 then has formed thereon a conductor 14 which is shown as composed of braided strands of wire which are bare strands. Each strand of wire is preferably laid singly, that is, not in parallel groups so that a structure which resembles two windings of opposed helices is formed by the conductor. It will be apparent, therefore, that at each crossing of the individual wire strands, a rather good electrical contact is maintained. This conductor is formed on the core 13 preferably for the entire length thereof which in the instant case is selected to be a quarter wavelength at a certain frequency is presented in FIGURE 2 as 7\a/4. An outer insulating covering 15 which is of a material similar to the material of the core and also impregnated is formed over the conductor 14 so as to completely cover the same. Over the covering 15 there is formed another conductor 16 which is laid on the covering for a distance indicated in the drawing as )\b/4 that represents a quarter of a wavelength at still another frequency.

The conductor and braid in this instance are identical to the previous conductor or braid that has been formed and that was designated 14. Over this second conductor 16 of shorter length, there is wound another insulating covering 17 for the full length of the antenna structure. Over this third insulating covering 17 a third conductor 18 is formed in the exact same manner as the original conductor was formed except in this instance the conductor 18 extends for a length much shorter than the other two conductors and is represented in the drawing as extending for a length Ac/4 which is a quarter-wave length at a certain predetermined frequency. At this point an outer sheath of material or complete covering 19 is placed over the structure to extend for the entire length thereof.

Tapering of the antenna structure to the general outline of FIGURE 1 may be readily affected by utilizing known techniques such as are used in the fishing rod industry. It will of course be apparent that by varying the thickness of the insulation and consequently the diameters of the conductors that the impedance of the various conductors may be varied as in effect the length-diameter ratio is being controlled.

The entire antenna formed as described above, is placed in an oven of appropriate temperature for the resin in use to cure the resin. The curing of the resin makes the entire antenna a homogeneous mass as a result thereof a perfect bond is maintained between the inner core and the successive layers onto the outer sheath. Accordingly, the various conductors are imbedded within the resin which prevents the access of air to the conductor.

By way of example let us assume that we wish to secure a broad coverage of frequencies such as from megacycles to 56 megacycles. For the sake of discussion we will assume that we can obtain a broad band effect for a certain resonant length of antenna conductor over a range of 10 megacycles. In order to produce a broad band effect on a whip antenna such as is disclosed herein, certain techniques may be utilized such as changing the length diameter ratio which directly affects the Q of the antenna and accordingly its broad band characteristic. To produce the first section therefore, we would install a section of braid which would resonate at say 29 megacycles, a second length of braid to resonate at about 37 megacycles, and a third length of braid to resonate at 45 megaoycles. The coverage, therefore, of each of the various lengths is presumed to be then 25 to 33 megacycles, 33 to 41' megacycles and 41 to 50 megacycles, which is well within the broad band assumption of 10 megacycles, a figure that is entirely feasible in a construction of this type. It will be apparent therefore that if one tunes across the band of 25 to 56 megacycles, the various portions of sections of the antenna will become resonant to the signal as described above.

To use this antenna, it is merely necessary to have a simple antenna coupling device which will accept a broad band of frequencies such as is shown in FIGURE 6. Accordingly an output tank circuit 20 may have inductively coupled thereto a link inductance 21 one side of which is grounded, either through a capacitance 22 or directly, and the other side of which is connected directly to the various conductors of the antenna. It has been found that this type of coupling is very feasible, since the impedance of the antenna is fairly constant over a large range and is extremely practical as antenna coupling switching is eliminated for band changes. Thus, the antenna of the invention may be simply and efficiently fed from a suitable transmitter.

It is obvious that many changes can be made in the embodiments described above and the embodiment is meant to be purely exemplary and it does not by any means comprise a complete list of substitution of elements that might be made which will be obvious to those skilled in the art.

I claim:

1. An antenna comprising as a radiation element concentric cylindrical conductors, a first inner conductor, a second conductor and a third conductor, said first, second and third conductors spaced from each other by a dielectric material, a resin coated fabric dielectric material enveloping said first, second and third conductors as an outer sheath, said first, second and third conductors being formed of two sets of helically arranged bare wire strands each set being wound in an opposing helical fashion, said first conductor being a quarter of a wave length at a first and highest operating frequency, said second conductor being a quarter of a wave length at a second operating frequency and said third conductor being a quarter of a wave length at a third operating frequency, the length of said third conductor being greater than half the length of the first conductor and a feed line coupling high frequency apparatus to one end of each of said conductors.

2. An antenna comprising a radiation element consisting of a plurality of conductors, each of said conductors being substantially a quarter wave length long at spaced operating frequencies, said frequencies being within a band represented by the wave length ratio 2 to 1, said conductors being circular in cross-section and formed of two sets of helically arranged bare wire strands, each set wound in opposing helical fashion so that one set crosses the other at a plurality of points, dielectric material, said conductors being embedded in said dielectric material, each of said conductors having one end thereof connected together to form a common feed-point.

UNITED STATES PATENTS References Cited in the file of this patent 1,495,537 Stafford May 27, 19% 2,112,287 Hansell Mar. 29, 1938 ,531 Hansell July 29, 1941 2,611,868 Marston et al Sept. 23, 1952 2,763,003 Harris Sept. 11, 1956 2,835,893 Braund May 20, 1958 FOREIGN PATENTS 505,877 Great Britain May 18, 1939 

